<select class="form-control" ng-model="vm.value">
  <option value="">-- 请选择城市 --</option>
  <optgroup ng-repeat="province in vm.provinces" label="{{province.label}}">
    <option ng-repeat="city in province.cities" value="{{city.code}}">
      {{city.label}} ( {{city.population|number}}万人 )
    </option>
  </optgroup>
</select>
<div>您选择的是：{{vm.value}}</div>
